At the present time, both intercity and out-of-state communications cables utilize both carrier and voice frequency transmissions. These cables are in the form of a multipair configuration and are used primarily for connecting central offices. Local Exchange Carriers (LEC) provide digital service to customers in common carrier systems, and, in North America, these systems operate at 1.544, 3.152 and 6.312 Mb/s data rates, and are commonly known as T1, TIC, and T2 systems, respectively. The cables most often are terminated at the customer's premises at a network interface (NI), where the transition from the outside plant (OSP) cable to the inside wiring is made. In general, the inside wiring is in the form of multiple twisted pairs of metallic conductors.
A dominant carrier system such as T1 is shown and described in an article in the Bell Laboratories Record, Vol. 40, No. 10, November 1962 at pages 358-363, and a Cable for T1 carrier use is shown in U.S. Pat. No. 4,262,164 of Nutt et al. In the T1 carrier cable, each twisted pair transmits data in one direction at the carrier rate and a compliment twisted pair transmits or carries data in the opposite direction. The T1 carrier outside plant design rules limit the maximum signal loss, which translates into cable distance between regeneration to 32 dB, and to 24 dB for an end span which originates or terminates at either the central office (CO) or the customer's building, or the equivalent. This insures that, for a properly designed cable, the transmitted signals will not interfere with the received signals. The lesser allowable loss for the end span takes into account the additional noise interference encountered inside or near the building.
It is common practice to separate the pairs of transmit and receive paths into different cables, or in different compartments of a cable divided by a conductive screen, as shown in the aforementioned Nutt et al. patent, or at the very least, to separate transmit pairs and receive pairs into multiple pair binder groups. The purpose of such separation is to minimize the signal interference at the cable ends of the receiving pair from the signal in the transmitting pair by having physical separation thereof and/or an element such as a shield or a screen interposed therebetween, to absorb the disturbing noise interference. Some cable designs have multiple individually shielded twisted pairs that provide isolation between every pair. However, it is unnecessary to shield every circuit or pair in the same signal direction and often these designs have impedance mismatch and increased or high attenuation making them unsuitable for most digital carrier signal transmission over significant distances.
The transition from an OSP cable to an inside wiring cable is made at the network interference (NI). It is often the case that the OSP cable is at the very limits of the aforementioned loss figure for the end span segment, or that it even exceeds these limits. Thus, further extension of the digital service beyond the NI can result in unacceptable signal-to-noise ratios leading to-transmission errors. The OSP cables often contain hundreds of pairs of conductors where a digital service to a customer's premises can often require sixteen (16) pairs or less. It is the practice to install multiple small or low pair cables which usually, however, because of their size, do not have effective binder group separation.
Many buildings typical of customers' premises have, in the interior thereof, drop ceilings that are spaced below a structural floor panel of concrete or the like. The drop ceiling supports light fixtures and other ceiling mounted hardware, and the space between the drop ceiling and the structural floor panel thereabove serves as a return-air plenum for the heating and cooling systems. In addition, this space or plenum is used for the installation and routing of communications, computer, and alarm system cables. The plenum represents a very real tire hazard in that it is, in effect, a duct having air currents therein. When a fire starts in, or reaches the plenum, it and the accompanying smoke can quickly spread throughout the entire floor or story of the building over which the plenum extends. The fire could travel along the length of the cables contained within the plenum, especially where the cable or wire insulation is flammable, such as in the case with many commonly used plastic insulators. Because of this possibility of catastrophic flame and smoke spread, the National Electric Code (NEC) prohibits the use of electrical cables within plenums unless they are enclosed in metallic conduits, and various local codes have been adopted embodying the strictures and requirements of the NEC Code. Inasmuch as metal conduits are difficult to route in plenums congested with other items or hardware, it becomes an extremely expensive proposition, both as to hardware and labor, to enclose the cables within conduits. As a consequence, there have been promulgated certain exceptions to the requirements for metal conduits in order to provide some relief from the prohibitive expense while still insuring adequate fire protection. Thus, the NEC and most local codes permit the use of flame resistant, low smoke producing cables without a metal conduit provided the cable has been tested and approved by a recognized reliable authority such as Underwriters Laboratories (UL).
What is needed and not, apparently, presently existent in the prior art, is a cable for use within the customer,s premises having characteristics that are a match, or at least do not clash, with the characteristics and parameters of T1 or other carder OSP cable; that affords an impedance match with such cable; that adequately maintains a separation between incoming signal bearing and outgoing signal bearing conductor pairs to insure, among other considerations, a low degree of cross-talk, and that is both fire retardant and low smoke producing while being less costly than most currently available cable.